This invention relates generally to the production of metal particles such as powders, and, more particularly, to the continuous production of rapidly cooled metal particles.
In many cases, rapidly cooled metallic particles or powders have highly desirable properties. Some metals, when cooled at rates of about 10.sup.5 .degree. C. per second or greater, retain the amorphous structure of the liquid into the solid state. Studies have shown that articles formed from such amorphous materials can combine strength, ductility, corrosion resistance, wearresistance, and other highly desirable features. Other metals rapidly cooled from the liquid state may not be amorphous, but nevertheless have very fine microstructures with an almost total absence of undesirable macroscopic segregation. Such rapidly solidified crystalline powders also exhibit highly desirable metallurgical, mechanical and chemical properties.
To make effective use of the properties of rapidly solidified metal particles, it is necessary in most instances that they be consolidated as by powder metallurgical techniques, into larger parts, and such techniques are known. As the processing of rapidly solidified particles into articles has become more commonplace, there has been an increased demand for large quantities of such particles. There therefore exists a continuing need for apparatus and techniques for producing quantities of rapidly solidified, controllably sized, metal particles of high purity.
Numerous techniques have been developed to produce rapidly solidified materials. The earliest techniques involved propelling a molten mass against a cold surface to "splat cool" the metal. Another approach was to place a droplet of liquid metal between two anvils and bring the anvils together. Both of these techniques are operable for laboratory scale work, but are not sufficiently continuous to produce the high volumes of particles necessary to meet current demands.
Larger volumes of rapidly solidified metal powders are ordinarily produced by atomization processes. As an example, in two-fluid atomization a stream of the molten metal to be atomized is impacted by a second stream of a high velocity fluid which causes droplet formation. The atomizing second fluid can be either a gas such as nitrogen or a liquid such as water. The maximum cooling rate of powders produced by gas atomization is typically about 10.sup.3 .degree. C. second, while water atomized particles have cooling rates of as high as about 10.sup.4 .degree. C. per second. Ultrasonic gas atomization is a newer technique wherein shockwave tubes produce gas pulses that atomize a metal stream, to yield cooling rates as high as 10.sup.6 .degree. C. per second. Many unsolved problems remain in adapting such techniques to commercial operations, including maintaining the desired purity of the powder material.
A promising technique for producing commercial quantities of metallic particles at high solidification rates employs centrifugal force to atomize liquids. In one such approach, a molten metal stream is directed toward the center of a rapidly rotating disk, and particles are produced as the liquid is atomized on the disk and thrown outwardly by centrifugal force. The atomized liquid droplets can then be rapidly cooled by convection, an impinging gas stream, or quenching into a liquid. One approach to providing the quench liquid is to place the liquid into a rapidly rotating cup. Centrifugal force causes the liquid to form a layer around the inner wall of the cup, to quench the particles as they impact the surface of the liquid layer. Current techniques of this type can produce cooling rates as high as about 10.sup.6 .degree. C. per second.
Centrifugal atomization techniques are promising candidates for commercial production of rapidly solidified powders, but as yet have not achieved their full potential for several reasons. First, no convenient, economical, continuous process has been proposed. Second, there is no approach for achieving cooling rates greater than about 10.sup.6 .degree. C. per second, even in very fine particles. Higher cooling rates offer the potential of producing new amorphous particles requiring such higher cooling rates, producing more rapidly solidified crystalline particles, and producing high purity, rapidly solidified particles of larger sizes and of shapes having greater utility in powder compaction techniques than do typical spherical particles.
There exists a continuing need for an improved apparatus and process for producing rapidly solidified metal particles, in commercial production quantities and in a continuous fashion. Such an apparatus would preferably be highly versatile, to allow the production of many types of metal particles, using a wide variety of quenchants, and in a variety of operating environments such as vacuum, liquids, or gases. The present invention fulfills this need, and further provides related advantages.